Glasses with high content of silver oxide

ABSTRACT

Glasses having a high content of silver oxide are described. Generally these glasses contain at least 20%, and up to as high as 70% silver oxide expressed as Ag2O. The glasses also contain a metal oxide of one of the metals, vanadium (V), tungsten (W), molybdenum (Mo), uranium (U) or manganese (Mn), and an oxide of one of the metalloids; viz. boron (B), phosphorous (P), germanium (Ge), arsenic (As), antimony (Sb), bismuth (Bi) or tellurium (Te). Alternatively, the glasses may contain in addition to the Ag2O component, a mixture of oxides of the above transition elements. The glasses are produced by forming a melt using silver in the form of AgNO3 to thereby enable formation of glasses which may have considerable quantities of Ag2O without precipitating the silver out in the metallic form. The glass compositions are characterized by low melting and low softening temperatures, high density, high refractive index, good infrared transmission characteristics and water resistance. Uses for the glasses include fiber optics, condensers, and glazes for substrates for microelectronic circuits.

Unite States Chvatal Q atent 91 GLASSES WITH HIGH CONTENT or v SILVER OXIDE Theodor L. Chvatal, Vienna, Austria [75] Inventor:

[73] Assigneez Owens-Illinois, Inc., Toledo, Ohio [22] Filed: Aug. 3, 1972 [21] Appl. No.; 277,803

Related US. Application Data [62] Division of Ser. No. 142,336, May 11, 1971, Pat. No.

OTHER PUBLlCATlONS v Weyl; W. A., Coloured Glasses, pub. by Soc. of Glass Tech., Sheffield, Eng, (1967 ed.), pp. 403-6.

[ 51 me. to, 1974 Primary ExaminerWinston A. Douglas {lssistant Examiner-Mark Bell Attorney, Agent, or FirmRichard B Dence", E. .l. Holler 5 7] 7 ABSTRACT Glasses having a high content of silver oxide are described. Generally these glasses contain at least 20%,

and up to as high as 70% silver oxide expressed as Ag O. The glasses also contain a metal oxide of one of the metals, vanadium (V), tungsten (W), molybdenum (M0), uranium (U) or manganese Mn), and an oxide of one of. the metalloids; viz. boron (B), phosphorous (P), germanium,(Ge), arsenic (As), antimony (Sb), bismuth (Bi) or tellurium (Te). Alternatively, the

. glasses may contain in addition to the Ag O component, a mixture of oxides of the above transition elements. The glasses are produced by forming a melt using silver in the form of AgNO to thereby enable formation of glasses which may have considerable quantities of Ag O without precipitating the silver out in the metallic form. The glass compositions are char-' acterized by low melting and low softening temperatures, high density, high refractive index, good infrared transmission characteristics and water resistance. Uses for the glasses include fiber optics, 'condensers, and glazes for substrates for microelectronic circuits.

19 Claims, No Drawings small amounts of silver, seldom exceeding 1%. The present invention, however, permits the formation of glasses having at least Ag O'and up to as high as 70% Ag O. These glasses are essentially silica-free and alkali-free.

In accordance with the present invention it is possible to form glasses that have many desirable physical characteristics including relatively low melting and softening temperatures, relatively high dielectric constants, low surface tension and semi-conducting properties. In addition, the glasses in the present application can be heat treated at relatively high temperatures to convert some of the silver present into the metallic state to bring about, for example, a reflecting surface or a conductive surface Glasses are amorphous compositions, prepared by super cooling. of a glass melt, whereby the transition from the liquid into the solid state has to be reversible.- While several different opinions exist on the structure of glasses, it is generally acknowledged that as far as the oxide glasses are concerned they consist mainly of oxygen polyhedra, in the center of which are generally located ions of relatively high charge and relatively small diameter, for example Si, B, P or Te; these are the so-called glass formers. The polyhedra are mutually connected via all oxygen ions, or via at least two, into a network or into chains. The network possesses an excess of negative charges, which are compensated for by network modifiers, that is, metal ions of relatively small positive charge andrelatively large diameters. These network modifiers are preferably the ions of the alkali metals and the alkaline earth metals. Some ions of somewhat smaller diameter and higher charge, for example Al, can also function as network modifiers, or can be built into the network itself. In the common glasses, the structural characteristics are mainly determined by the glass formers, with network modifiers generally considered as exhibiting a lesser influence on the ultimate nature of the glass.

In the last decade, a completely new group of glasses was found with very interesting properties. See, for example, J M. Stevels, Philips TechnischeRundschau 22 (1961). These are the so-called invert glasses. The specific feature of these glasses is that the volume percentage of oxygen polyhedra is below 50 volume in contrast to the conventional or common glasses where the oxygen polyhedra, together with the glass'formers, furnish the main portion of the volume. There are no continuous networks or chains in the invert glasses.

The glassy state arises on cooling, more particularly'by means of a mutual interference of the crystallization. For this purpose at least two network modifiers have to be present, the size and charge of which should be different. i

. The properties of the invert glasses are dramatically different from he properties of the other glasses. Unlike the case of conventional glasses, in invert glasses the network modifiers have a strong influence on the nature of the glass. These glasses have a more open structure and therefore possess low softening temperatures and high expansion coefficients. They also have relatively low viscosities, steep viscosity curves and low dielectric loss tangents.

It is therefore an object of the present invention to provide new glasses containing silver oxide which have desirable properties and characteristics.

It is a further object of the present invention to provide a method for making new glasses containing silver oxide.

It is still another object of the present invention to provide products formed from new glasses having a high silver oxide content.

It is a further object of the present invention to provide a new group of glasses with new properties thereby making them suitable fora wide variety of purposes.

In achieving the foregoing objects, one feature of the invention resides in new silica-free glasses in which Ag O is present in a relatively large amount, i.e.,- 20% or more.

In another feature of the invention these new glasses may contain up to Ag O and are essentially alkalifree. In the present invention, the Ag O where the Ag has an ionic radius of 1.26 A (Pauling), is combined with other metal ions, .preferably with metal ions of the so-called metal groups and with another selected metalloid ion. For purposes of this invention, the metal ions of the metal group, V, W. Mo, U and Mn have ionic radii between 0.5 and 0.8 A. These dimensions for ionic radii refer primarily to the highest valence state of these metal elements. In other words, Ag O is combined with oxides of metals with relatively small ionic radius and high valence (at least over 4). The third component of the glass systems of this invention is chosen from the group of metalloids; viz., B, P, Ge, As, Sb, Bi and Te. Here the ionic radii are still smaller and the valences are 3 to 5.

A further feature of the invention resides in novel glass compositions containing at least 20% by weight Ag,,O and 10 to 70% by weight of an oxide of a metal selected from the group consisting of vanadium, tungsten, molybdenum, uranium and manganese and further containing 10 to 70% by weight of an oxide of a metalloid selected from the group consisting of boron, phosphorous, germanium, arsenic, antimony, bismuth and tellurium.

A still further feature of the invention resides in novel glass compositions containing at least 20%by weight Ag O and 30% and by weight of'a two-component mixture ofmetalloid oxides wherein the metalloid is selected from the groups consisting of boron, phospho rous, germanium, arsenic, antimony, bismuth and tellurium.

Surprisingly, it has been determined that the glasses of this invention containing up to 70% of Ag O are stable in these combinations. In the prior art, it was the common practice-to introduce the Ag O into glasses only as an addition agent for various functions, for example, as nucleant or as an indicator for ionizing radiation. In other developments, larger amounts of silicon dioxide were present in glass but with limitations on the upper amounts that could be tolerated. In a publication of Provance, J.D., and Wood, D.C., Molybdenum Phosphate Glasses Containing Ag O and K 0, J. Amer. Ceram. Soc. Vol. 50 (I967) (10) pp. 5l6-520,

glasses with Ag O up to 23 wt. are also mentioned. According to the present invention, however, it is possible to produce stable glasses with up to 70 wt. Ag O. The advantages of the present invention are obtained through two means: (1') the choice of the other metal ions in the system, and (2) the utilization of AgNO as raw material for the glasses. It has been determined that Ag O forms thermally stable compounds with the oxides of the selected transition metals. In contrast to these stable compounds, all oxide containing compounds of silver, e.g., Ag O, and AgOH, AgCO and AgNO show the tendency to decompose above 300C, whereby metallic Ag is formed. 300

Therefore a further feature of the invention resides in the use of silver nitrate as the raw batch material for the formation of Ag O in the glass. AgNO is very low melting, at approximately 200C., and serves as an excellent melting aid in the synthesis of glasses of high silver content. By following the procedure of using Ag- NO a melt of good reaction with the other components; viz. good solid-liquid contact is assured. The Ag O, formed through the reaction of the other components with AgNO gradually decomposes AgNO Traces of N however, remain in the melt and conteract the thermal decomposition of Ag O. In accordance with the invention the melts for synthesis of the glasses may be heated to temperatures up to l,0O0C. without reducing the silver to metal.

Another advantage in the use of silver nitrate resides in making possible combinations with other oxides which sublime at low temperatures,for example, combinations with As O Sb O and TeO Very low melting glasses can be obtained with these glass formers.

Finally, it has been determined that the AgNO has yet another function. If oxides of the transition metals are to form glasses, then the ions should be present in their highest valence, because then the requirement of the smallest ion radius and the highest valence is met. ln this respect, the silver nitrate melt is very oxidizing and for example, is capable of oxidizing Mn to Mn.

The following ternary glass systems are capable of forming relatively extensive glassy regions as will be apparent from the illustrative compositions:

illustrative of this invention are the'following compositions in weight percent:

' wo -60 P205 12-40 Ag O 20-05 Teo 15-65 1 ,0, 10-40 Ag o -70 v.0, 10-60 A5201, 12-40 Ag,0 20-60 -Continued W0 l0-60 AS 0 l 5-50 A ;;,o 20-5 5 M00 10-60 A5 0 1 5- 3 5 Ag O 20-60 TeO 1 0-60 A5 0, l 0-40 Ag O 20-50 Bi O l 0-60 B 0, 20-65 Ag O 20-50 GeO 1 5-5 5 B 0 20-50 A g O 20-45 Te0 10-60 2 :1 l 5-50 gt 20-e0 In the following ternary glass systems glass forming regions of medium extent were found:

Included within this second group of ternary glass systems and illustrative of the present invention are the following compositions in weight percent:

All glasses of the invention possess a common property of relatively low softening temperature. This is primarily affected through the Ag O, because the latter actually acts in .theseglasses like an alkali oxide, but has the great advantage over use of alkalies in glasses because Ag O is difficult to dissolve .in water. This is true not only for Ag O, but also for the other oxygen containing silver compounds, for example, AgOl-l. Despite their low softening temperatures, these glasses therefore have a good water and moisture resistance. In

addition, there is a good stability against bases (lye) (except ammonia) and also against HCl.

As will be apparent to persons skilled in the art that the properties of the glasses willvary because of the different possible combinations with ions of the transition metals and also with glass forming ions.

Depending upon the combination of oxides, the novel glasses possess very high specific weights, high refractive indices and, in some cases, very high dielectric constants. These properties are dependent on the atoms, and it is possible to combine with the heavy Ag in the glass other atoms of high atomic weight even in larger quantities, for example W, U, Bi, Sb and Te. It generally can be concluded that the glasses of the in vention containing P and As O as well as TeO as a glass forming oxide have low softening temperatures, low viscosities and relatively steep viscosity curves. In contrast, glasses with B 0 as the glass forming oxide show inverse properties, although even here the softening temperatures are relatively low.

Because of their viscosity characteristics the glasses of this invention may be technologically utilized for casting of seed free glasses. With the new glasses of high refractive index new fiber optics can be made. Glasses of low softeningtemperature, low viscosity, high dielectric constant and high electrical resistivity (for example, glasses in the systems TeO P- O Ag O) can preferably be utilized as amatrix for 'dispersed BaTiO powder. Even matched density and On the other hand,, at high temperatures and in contact with metals, the Ag O can be reduced to metallic Ag. Thus, silver can diffuse into the metals or can form alloys with the substrate. In this way a good adhesion is achieved even to metals that are hard to glaze, for example, gold.

The thermal expansion coefficient of glasses according to this invention is related to their structure. The coefficient is relatively high and increases as the Ag O content is increased. In glasses formed with P 0 and As O as glass formers,-the expansion coefficient is higher than with glasses that contain B 0 as the glass forming oxide. The expansion coefficient is more similar to that of the metals. Thus, by following the teachings herein, it is possible to produce glasses which are close to metals in thermal expansion characteristics,

but which possess an advantage over prior art glasses in that they are stable glasses. High expansion coefficient is often desirable or even necessary for certain applications such as matching metals for bonding to metals. Heretofore, some glasses. have been obtained possessing the desired expansion properties by the addition of Na O to the glass. Unfortunately, the durability of the glasses is decreased with increasing Na O content. With the present invention, however, glasses of good durability and high coefficient of expansion are obtainable.

Since the invention allows for manufacture of glasses of high metal oxide content, it is possible to produce colored glasses, for example by addition of W0 M003, Miio,, U308, TeO and sip, or, by addition of V 0 tomelt glasses that transmit only in the infrared region.

Especially noteworthy are the electrical properties of the glasses of this invention. The capability of achieving a high dielectric constant has already been pointed out. But also the specific electrical resistance can be widely varied depending upon the composition. lllustratively, it can lie between 10 to 10 Ohm-cm, and persons skilled in the art with the information contained herein will be able to tailor the compositions of the glasses for an intended purpose or function.

When P 0 and AS203 are used as glass formers, an addition of V 0 W0 or M00 will affect a strong reduction of electrical resistance, especially in combination with a higher content of Ag O. Through these combinations the invention affords the production of semi-conducting glasses. The temperature coefficient of the specific electrical resistance is strongly negative. The activation energies, E, of these semi-conducting glasses are very low and lie between approximately 0.3

to 0.5 e'V. Since the crystallization tendency in these semi-conducting glasses of the invention can be more or less pronounced, depending upon composition, it is possible to use them as elements forswitching devices; viz. elements for which the resistance changes discontinuously at a certain threshold voltage. Even thin conducting layers can be put on other materials by use of these semi-conducting glasses of this invention.

The following examples are illustrations of how to prepare glasses of the invention:

Example 1,:

73 g. AgNO are mixed with 32 g. tungsten oxide, and thickened with a phosphoric acid. After melting at approximately l,000C. a yellow, translucent glass of the composition 30% W0 20% P 0 and 50% Ag O, is obtained. This glass composition 'is annealed at 300C.

Example 2:

54 g. AgNO are mixed with 52 g.-Te0 and thickened with 20.5 g. ofa 75% phosphoric acid. After melting at 600C a yellow glass of the indicated composition is obtained: 52% T602, 11% P 0 37% Agro. This glass is annealed at 230C. Example 3:

57 g. AgNO are mixed with 20 g. U0 and thickened -with 73 g. of a 75% phosphoric acid. After melting at annealed at lC.

. Example 5:

ing at 950C a violet glass of the composition: 20%

550C. A reddish-brown glass of the following composition is obtained: 10% W 35% AS203, 55% Ag O. This glass is annealed at 210C. Example 9:

88 g. AgNO are mixed with 10 g. MoO;,.and 33 g. AS203 as described in Example 7 and melted at 500C. A greenish glass which has the indicated composition: 10% M00 30% As O 60% Ag O is obtained. This glass is annealed at 270C.

Example 10: g

58 g. AgNO are mixed with 30 g. U0 and 33 g. As O as described in Example 7 and melted at 550C. An orange glass of the composition 28% U 0 35% As- O 37% Ag O is obtained. This is annealed at 300C Example 11:

73 g. AgNO are mixed with 30 g. TeO and 33 g. A 0 as described in Example 7 and melted at 400C. A reddish glass having the composition 30% TeO 30% A5 0 40% Ag O is obtained. This is annealed at 210C.

Example 12:

66 g. AgNO are mixed with 20 g. TeO and 35 g.

V 0 as described in Example 7 and melted at 500C.

A brown glass of'the composition 35% V 0 20% 40 TeO 45% Ag O is obtained. This glass composition is annealed at 150C.

Example 13:

58 g. AgNO 20 g. Bi O and 71 g. H 130 are dry mixed. thickened with 100ml. H O, dried out again, and melted at 700C. One obtains a yellowish glass of the following composition: 20% Bi O 40% B 0 40% Ag O. This glass is annealed at 310C.

Example 14:

58 g. AgNO 20 g. GeO and 71 g. H 80 are prepared as in Example 13 and melted at 900C. One obtains a yellowish glass of the composition 20% (1e0 40% B 0 40% Ag O. This glass is annealed at 430C.

Example 15:

43 g. AgNO 50 g. TeO and 53 g. H BO are prepared as in Example 13 and melted at 600C. One obtains a yellowish glass of the composition: 40% TeO 30% B 0 30% Ag O. This glass is annealed at 330C. Example 16:

58 g. AgNO 15 g, MnO and 80 g. H 80 are prepared as in Example 13 and melted at 900C. One obtains a violet glass of the composition: 15% Mn O-,, 48% 8 0 37% Ag O. This glass is annealed at 450C. Example 17:

88 g. AgNO are mixed dry with 44 g. H BO and thickened with 28 g. of a 75% H PO This glass is melted at 800C. A yellowish glass of the following composition is obtained: 15% P 0 25% B 0 60% Ag O. This glass is annealed at 380C.

All the glasses of Examples ll2 can be melted in unglazed porcelain crucibles, Pythagoras"-crucibles or A corundum crucibles. The borate glasses are-somewhat more aggressive and should preferably be made in corundum crucibles.

The following Tables contain additional representative glasses of the present invention together with phys ical property data:

' TABLES Legend for the Tables:

T Softening Temperature T Melting Temperature p Specific Electrical Resistance (Ohm.cm)

a Linear Coefficient of Expansion (cm/cm.C)

e Dielectric constant TABLE 1 Glass Composition. Wt. 7r T "C T C p or & W0 W0 P 0 Ag O A 10 35 55 200 360 6.3Xl0 188 B 22 63 250 400 3.0 10 256 C 3O 50 240 400 5.6X10 206 D 30 20 50 380 560 2.9X10 203 E 30 30 40 330 580 3.6Xl0 178 F 45 430 670 l.5 l0 135 o 57 15 28 530 740 78x10" 130 18.3

TABLE 2 Glass Composition, Wt. T,; C T "C p 11x10 e TeO TeO P 0 Ag O A 30 35 280 470 1.4-)(10 168 B 47 26 27 280 470 1.0X10 210 C 65 15 20 310 480 3.5 10 186 20.4 D 52 11 37 250 390 5.5 10 247 19.4 E 30 15 55 200 360 2.1X10 246 F 15 20 65 190 420 1.8)(10 262 G 29 26 250 430 9.7X10" 205 U 0 U 0 P 0, Ag O A 20 40 40 240 450 30x10" 206 B 28 25 47 Y 310 470 9.l l0 199 C 35 32 33 400 630 1.7X10 162 D 25 25 440 730 5.1)(10 151 18 8 81,0, B1203 P 0 Ag O A 15 30 190 400 2.9Xl0 229 20.5 B 20 40 40 250 440 3.8X1O 192 C 30 32 38 280 570 1.1X10 184 15.1 D 45 35 20 340 620 2.7 10

11 Batch materials used to form the Compositions of the present invention can vary widely with the exception that silver nitrate, AgNO is the preferred source of silver. The foregoing examples show illustrative batch 6. A glass composition as defined in claim 1 consisting essentially of the following ingredients based on the total oxide composition:

materials and equivalent components will be apparent 5 Component Weight Percent to those having ordinary skill in the art.

What is claimed is: 5,0,. 15-35 1. A glass composition consisting essentially of the 32 2 following components, expressed in weight percent of e the total oxides in the compositions, selected from the 10 a group consisting of: y A glass compos1t1on as defined 1n claim 1 cons1stsilverloxide 2O 70% 10 to 70% of an o'xide of a ing essentially of the following ingredients based on the transitional metal selected from the group consisttotal oxlde compo-Simon: ing of tungsten, uranium and manganese and 10 to v 70% of an oxide of a metalloid selected from the Cmnponen weigh Perm" group consisting of boron, phosphorous, germa- T602 04,0 nium, arsenic, antimony, bismuth and tellurium; A5 0 -40 B. silver oxide -70% and 30 to 80% of -a two- A810 component mixture of oxides of a metalloid se- 0 I lected from the group conslstmg 9 phos 8. A glass composition as defined in claim] consistphorous, germamum, arsemc, ant1mony, b1smuth mg essentlally of the followmg ingredients based on the and tenunum and Y A total oxide com osition C. silver oxide 20 70%, to 10 to 70%, of molybdenum p oxide or vanadium oxide and 10 to 70% of an oxide of a metalloid selected from the group consisting of Component Percemage boron, germanium, arsenic, antimony, bismuth and B90 10-60 tellurium. I 2 6 58*? 2. A glass composition as defined in claim 1 consisting essentially of the following ingredients based on the total oxlde composmon: 9. A glass composition-as defined in claim 1 consisting essentially of the following ingredients based on the Component Weight Percent total oxide composition:

n 1040 Component Weight Percent P,o 12-40 22 204,5 Goo 15-55 B 0 20-50 v Ag O 20-45 3. A glass composition as defined in claim 1 c'onsisting essentially of the following ingredients based on the 40 I total Oxide composition: 10. A glass composltion as defined 1n cla1m l cons1sting essent1ally of the followmg 1ngred1ents based on the total oxide composition: Component Weight Percent T602 [54,5 Component Weight Percent S6 838 E8; 2:28 Ag O zo-oo 4. A glass composition as defined in claim 1 consisting essentially of the following ingredients based on the 11. A glass composition as defined in claim 1 consisttotal oxide compositions: ing essentially of the following ingredients based on the total oxide composition:

Compqnem weigh Percent 5 C o t Weight Percent A glass Composition as defined in claim 1 consist '12. A glass composition as defined in claim 1 consisting essentially of the following ingredients based on the ing essentially of the following ingredients based on the total oxide mp i i n: total oxide composition:

A Component Weight Percent Component Weight Percent wo, [0-60 sb,o, 10-45 Asp 15-50 P 0 30-50 Ag O 20-55 A ,0 20-o0 13. A glass composition as defined in claim 1 consistp n t Weight Percent ing essentially of the following ingredients based on the total oxide composition: 2 :38 5:23 Ag O 25-40 5 Component Weight Percent vBizoa 045 17. A glass composition as defined in claim 1 consistzzg 33:28 ing essentially of the following ingredients based on the 2 total oxide composition:

14. A glass composition as defined in claim 1 consist- C t w h ing essentially of the following ingredients based on the onlponen mg Perm total oxide composition: Mn,0, -20 B 0 40-60 555 0 @75 5 Component Weight Percent Mn O, 10-25 p 0, 35-60 Ag O -55 18. A glass composition as defined in claim 1 consist- 2O ing essentially of the following ingredients based on the 15. A glass composition as defined in claim 1 consisttotal oxide composition:

ing essentially of the following ingredients based on the total oxide composition:

. Component Weight Percent Component Weight Percent P 0, 10-20 V 0 15-50 B 0 25-40 Teo 20-60 I ggo 45-60 Ag O 20-45 16. A glass composition as defined in claim 1 consisting essentially of the following ingredients based on the total oxide composition:

19. An article formed of the composition of claim 1. 

1. A GLASS COMPOSITION CONSISTING ESSENTIALLY OF THE FOLLOWING COMPONENTS, EXPRESSED IN WEIGHT PRECENT OF THE TOTAL OXIDES IN THE COMPOSITIONS, SELECTED FROM THE GROUP CONSISTING OF: A. SILVER OXIDE 20-70%, 10 TO 70% OF AN OXIDE OF A TRANSITIONAL METAL SELECTED FROM THE GROUP CONSISTING OF TUNGSTEN, URANIUM AND MANGANESE AND 10 TO 70% OF AN OXIDE OF A METALLOID SELECTED FROM THE GROUP CONSISTING OF BORON, PHOSPHOROUS, GERMANIUM, ARSENIC, ANTIMONY, BISMUTH AND TELLURIUM; B. SILVER OXIDE 20-70% AND 30 TO 80% OF A TWO-COMPONENT MIXTURE OF OXIDES OF A METALLOID SELECTED FROM THE GROUP CONSISTING OF BORON, PHOSPHOROUS, GERMANIUM, ARSENIC, ANTIMONY, BISMUTH AND TELLURIUM, AND C. SILVER OXIDE 20-70%, TO 10 TO 70% OF MOLYBDENUM OXIDE OR VANADIUM OXIDE AND 10 TO 70% OF AN OXIDE OF A METALLOID SELECTED FROM THE GROUP CONSISTING OF BORON, GERMANIUM, ARSENIC, ANTIMONY, BISMUTH AND TELLURIUM.
 2. A glass composition as defined in claim 1 consisting essentially of the following ingredients based on the total oxide composition:
 3. A glass composition as defined in claim 1 consisting essentially of the following ingredients based on the total oxide composition:
 4. A glass composition as defined in claim 1 consisting essentially of the following ingredients based on the total oxide compositions:
 5. A glass composition as defined in claim 1 consisting essentially of the following ingredients based on the total oxide composition:
 6. A glass composition as defined in claim 1 consisting essentially of the following ingredients based on the total oxide composition:
 7. A glass composition as defined in claim 1 consisting essentially of the following ingredients based on the total oxide composition:
 8. A glass composition as defined in claim 1 consisting essentially of the following ingredients based on the total oxide composition:
 9. A glass composition as defined in claim 1 consisting essentially of the following ingredients based on the total oxide composition:
 10. A glass composition as defined in claim 1 consisting essentially of the following ingredients based on the total oxide composition:
 11. A glass composition as defined in claim 1 consisting essentially of the following ingredients based on the total oxide composition:
 12. A glass composition as defined in claim 1 consisting essentially of the following ingredients based on the total oxide composition:
 13. A glass composition as defined in claim 1 consisting essentially of the following ingredients based on the total oxide composition:
 14. A glass composition as defined in claim 1 consisting essentially of the following ingredients based on the total oxide composition:
 15. A glass composition as defined in claim 1 consisting essentially of the following ingredients based on the total oxide composition:
 16. A glass composition as defined in claim 1 consisting essentially of the following ingredients based on the total oxide composition:
 17. A glass composition as defined in claim 1 consisting essentially of the following ingredients based on the total oxide composition:
 18. A glass composition as defined in claim 1 consisting essentially of the following ingredients based on the total oxide composition:
 19. An article formed of the composition of claim
 1. 